The present invention is generally directed to improvements in color television receivers. It is particularly directed to a set-up circuit for establishing a predetermined voltage at the outputs of video amplifiers when the receiver's G2 voltage is being set for proper cut-off of the receiver's electron guns.
It is conventional practice in the television industry to adjust the G2 voltage of a receiver's CRT (cathode ray tube) so that the electron gun with the highest cut-off point is barely emissive. Prior to making such adjustment, the output voltages of each of the three video amplifiers are set to a common voltage level. FIG. 1 illustrates a typical arrangement for setting the output voltages of the three video amplifiers.
As shown, the video amplifiers each include a transistor operating as a common emitter amplifier for its color-difference input and as a common base amplifier for its luminance input such that an amplified video output is taken at the collector of each transistor. The output of the red amplifier drives a cathode 10 in the receiver's CRT. The collectors of the green and blue amplifiers are also coupled to other cathodes (not shown).
In normal operation, the emitter resistors of the three amplifiers are coupled via a switch 12 to the emitter of a transistor 14 which receives the Y or luminance signal at its base for mixing with the color-difference signals received by the three amplifiers.
For G2 set-up, the switch 12 is coupled to a grounded resistor 16. The purpose of this arrangement is to cause the collector of each amplifier to be at the same D.C. voltage level when the G2 voltage is adjusted for proper cut-off.
The problem with this arrangement, and others like it, is that the collector voltage of each amplifier is also a function of its base voltage. The base drive for each amplifier is derived from the output of a color-difference demodulator. Because the demodulator's outputs tend to drift and to be unequal, it is difficult to ensure that the collector voltages of all three amplifiers are at the same level when the switch 12 is in its illustrated position.
This problem is of greater significance for so-called RGB systems in which the output amplifiers receive R, G and B inputs rather than color-difference inputs. These R, G and B inputs each have a D.C. level which will vary as the brightness control is adjusted. Hence, the outputs of the R, G and B amplifiers will vary accordingly, thereby making it more difficult to obtain proper G2 set-up. Many other types of G2 set-up circuits suffer from the same problem, therefore preventing the G2 voltage from being properly set.